Secondary batteries (for example, lithium ion battery) have advantages of high energy density, high power density, long cycling life, and long storage life, etc., have been widely applied in portable electronic devices, such as mobile phones, digital cameras, portable computers and so on, and also have a broad application prospect in electric transportation vehicles, such as electric vehicles, electric bicycles and so on, and large and medium electric equipment such as energy storage facilities and so on. The secondary battery has become a key solution for solving worldwide problems such as energy crisis and environment pollution.
When the secondary battery is overcharged, the electrolyte solution in the secondary battery will decompose so that the internal pressure of the secondary battery increases, which causes fire and explosion to the battery. In order to prevent the fire and explosion caused by the increase of the internal pressure, an external short-circuit unit, and a fuse placed between the positive electrode plate and the positive electrode terminal. When the internal pressure of the secondary battery increases, the external short-circuit unit connects the positive electrode plate with the negative electrode plate of the electrode assembly into a short circuit providing a great current, which in turn fuses the fuse between the positive electrode plate and the positive electrode terminal, thereby cutting off the charging state of the electrode assembly.
However, the structure as described above reduces the ability of the secondary battery to cope with a high temperature environment. For example, in the hot-box test, the probability of thermal runaway of the secondary battery with the above structure significantly increases. It is because: in the high temperature environment, not only the internal pressure of the secondary battery increases with the decomposition of the electrolyte solution at the high temperature, but also the inner resistance of the electrode assembly increases; at this time, if a short circuit is formed by connecting the positive electrode plate with the negative electrode plate of the secondary battery by the external short-circuit unit, the electrode assembly will generate more heat due to the increase of the internal resistance and the presence of the short circuit current, which worsens risks of fires and explosion of the secondary battery. In other words, the external short-circuit unit in the related art can only solve the overcharge problem of the secondary battery at normal temperature, while increasing the probability of thermal runaway at high temperature, reducing the safety of the secondary battery.